I recently faced the same dilemma. I have 300 BLK SBR with a 10.5" barrel. I plan to use it largely for hunting, as I have other guns for personal protection. That doesn't mean it couldn't serve in a defensive role, that just isn't what I am optimizing it for. I went with a Leupold Mark AR Mod 1 3-9x40 with mil dot reticle. I have one on another gun and really liked it. I also have the Mark AR Mod 1 1.5-4x20 with duplex reticle on another gun.
I chose the 3-9x40 for the 300 BLK because I find that to be the most useful range of magnification for most of the hunting situations I find myself in. I went with the mil dot reticle because it gives me nice hold points for shooting subs or for shooting supers at extended ranges. I have the rifle zeroed at 100 yards with 110 grain supers. With that same zero, the bottom mil dot is the hold point for 220 grain subs at the same range.